Difference between include(), require(), include_once(), and require_once() in PHP

include(), require(), include_once(), and require_once() are all PHP functions used to include and evaluate the specified file during the execution of a script. However, there are some key differences between them:

1. include(): If the specified file is not found, PHP will generate a warning and continue executing the script.
2. require(): If the specified file is not found, PHP will generate a fatal error and halt the script execution.
3. include_once(): This function is similar to include(), but ensures that the specified file is included only once to prevent multiple inclusions.
4. require_once(): This function is similar to require(), but ensures that the specified file is included only once to prevent multiple inclusions. If the file is not found, PHP will generate a fatal error.

It is generally recommended to use require() or require_once() when including files that are essential for the script to function correctly, and include() or include_once() for files that are not crucial and can be skipped without halting the script execution.